This invention relates to 2-phenyl-5-(4'-trans-cyclohexyl) phenyl pyrimidine derivatives, and more particularly to novel liquid crystal compositions including 2-phenyl-5-(4'-trans-cyclohexyl) phenyl pyrimidine derivatives suitable for use in electro-optical displays.
Liquid crystal display devices utilize electro-optical effects possessed by liquid crystals. The liquid crystal materials used in these devices have a nematic phase, a cholesteric phase and a smectic phase. The most widely used display mode uses liquid crystal materials in the nematic phase and include the dynamic scattering type (DSM), guest-host type (G-H), twisted nematic type (TN), super-twisted nematic type (STN), super-twisted birefringence (SBE) modes and the like. The driving systems used for these liquid crystal display devices include the static driving system, time driving system (Dynamic Driving System), active matrix driving system, two frequency driving system and the like.
Liquid crystal display devices have several advantages, particularly when compared with conventional light emission type displays including LED devices, EL devices and CRT devices. The devices are small in size and can be made thin, can be driven at low voltage with low power consumption, and show good compatibility with LSI and simple driving circuits. The liquid crystal material is a light receiving element so that when a liquid crystal display is viewed over a long time, eye strain does not occur.
In view of these benefits, liquid crystal display technology has been applied to watches, cameras, electronic counters, audio equipment, automobile dashboard indicators, electronic games, telephone equipment, measuring devices, and the like. More particularly, liquid crystal display devices have also been utilized recently in other devices including displays which require high resolution and many pixels.
The predominant liquid crystal display device is a TN type utilizing a time sharing driving system. However, the maximum number of scanning lines is about 200 and attempts to increase this number have been unsuccessful. In order to increase the number of scanning lines, STN mode liquid crystal display devices and TN mode liquid crystal display devices driven by active matrix driving systems have been used. The STN mode is currently utilized in liquid crystal display devices in personal computers and word processors, while TN mode devices driven by active matrix driving systems are predominantly utilized in color televisions. Thus, liquid crystal display devices continue to attract attention as potentially replacing cathode ray tubes. As a result, liquid crystal display devices have been applied in various areas and it is likely that their use will be broadened further.
For practical use, liquid crystal compositions must possess the following characteristics:
1. The liquid crystal materials must be colorless and thermally, optically, electrically and chemically stable;
2. Have a wide nematic temperature range; and
3. The driving voltage be low.
Many liquid crystal materials possess the first of the above-desired properties, however, no single compound satisfies all of the remaining characteristics. Thus, liquid crystal compositions are formed of several different nematic liquid crystal compounds or liquid crystal compositions are obtained by mixing liquid crystal compounds with non-liquid crystal compound to obtain the desired properties.
In order to satisfy the property of a wide nematic range, a liquid crystal compound having an N-I Point as high as possible and an N-I Point as low as possible is required. In general, the liquid crystal compositions are eutectic mixtures in order to reduce the lower limit of the nematic liquid crystal temperature range as low as possible. The composition of the eutectic mixture composition can be made by setting the ratio between each of the components to a particular composition ratio.
In general, to raise the upper limit of the temperature range, compounds having a high N-I Point are used. These compounds include, for example: ##STR2##
In order to decrease the driving voltage of a liquid crystal display device, it is necessary to decrease the threshold voltage. However, the following relationship exists between the threshold voltage (V.sub.th), the thickness of the liquid crystal layer (d), the spray elasticity constant (K.sub.11), the twist elasticity constant (K.sub.22), the bend elasticity constant (K.sub.33), dielectric constant anisotropy (.DELTA..epsilon.) and the dielectric constant in a vacuum (.epsilon..sub.0): ##EQU1##
Thus, in order to decrease V.sub.th, a liquid crystal compound having a large dielectric constant anisotropy (.DELTA..epsilon.) and a small elasticity constant is required. However, conventional compounds having a high N-I Point and large .DELTA..epsilon. have very high elasticity constants which adversely effect V.sub.th.
Accordingly, it is desirable to provide an improved liquid crystal material and composition having a wide temperature range and a low threshold voltage.